1. Field of the Invention
The present invention relates to a semiconductor memory device including read-only memory (ROM) cells, and more particularly, to the improvement of a sense amplifier in a semiconductor memory device.
2. Description of the Related Art
ROM devices are broadly divided into a nonvolatile ROM device such as an erasable programmable ROM (EPROM) device and an electrically erasable programmable ROM (EEPROM) device, and a mask ROM device.
In an EPROM device or an EEPROM device, binary data. "0" or "1" corresponds to the low or high threshold voltage of a transistor, which is determined by introducing carriers (electrons) into a floating gate by a tunnel effect.
Similarly, in a mask ROM device, binary data "0" or "1" of a memory cell corresponds to:
the presense or absense of an enhancement type transistor;
the low or high threshold voltage of a transistor;
the depletion type or enhancement type of a transistor; or
the presense or absense of a contact window (throughhole) which connects a transistor to one bit line.
A typical ROM device includes a memory cell array formed by ROM memory cells, a digit line for receiving read data from a selected one of the ROM memory cells, a sense amplifier for sensing a voltage at the digit line to generate a sense voltage signal, and an output buffer for outputting an output signal in response to the sense voltage signal.
A prior art sense amplifier includes a bias circuit for amplifying the voltage at the digit line, a reference voltage generating circuit for generating a reference voltage signal, and a differential amplifier for amplifying the difference in potential between the output signal of the bias circuit and the reference voltage signal (see: S. Tanaka et al., "A Programmable 256K CMOS EPROM with On-Chip Test Circuits", IEEE International Solid-State Circuits Conference, pp. 148-149, 1984). This will be explained later in detail.
In the above-described prior art sense amplifier, however, one reference voltage generating circuit is provided for each sense amplifier. If the number of sense amplifiers is increased, the integration is reduced, and also, the power dissipation is increased due to the current flowing through the reference voltage generating circuits.